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math

graham7773

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how much difference will 5mm extra in a tyres profile make to the speedo reading? Or will 5mm just make the reading accurate at 100kph. Just to be clear, I am talking about going from 205 x 65 to 205 x 70.
 

hakhawk

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the 65 and 70 arent mm measurements. theyre percentages of the tyre width.

Tire size calculator

punch the tyre specs in there and itll tell you how far out your speedo will be.
 

GEN3RATD

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^^^ correct, as i understand it 245/30 = 30% of 245 would be the sidewall height.
i think this is why cars that have staggered tyre sizes like 245/35 fronts and 275/30rears have the 30 aspect ratio on the rear so that it balances out? as the ratio would be similiar as the rears are wider.
not sure though
 

soop

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245 is the width of the tyre in MM.
the 30 is sidewall height.

IRC, 30% Of 245mm.

245 / 30 = 8.1

which would mean the side wall is 8.1cm tall from rim. Sounds about right to me.
 

hakhawk

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245 is the width of the tyre in MM.
the 30 is sidewall height.

IRC, 30% Of 245mm.

245 / 30 = 8.1

which would mean the side wall is 8.1cm tall from rim. Sounds about right to me.
73.5mm if you do the math correctly :p
 

00vxsc

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we all know that 10% of 245 is 24.5mm. 24.5mmx3(10%) is 73.5mm
 

dunky

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In answer to your particular tyre size enquiry - 205/65 will give you 133.25mm sidewall . 205/70 will give you 143.5 mm. Therefore your tyre/wheel overall diameter will increase by 20.5mm. I don't know if this will increase or decrease your speedo reading. Perhaps someone else might have that knowledge
 

Cheap6

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The speedo. will read lower than actual speed. Circumference is pi X dia. so increasing the dia. increases the distance travelled per revolution of the wheel. The speedo measures the number of rotations per unit of time => greater distance travelled per unit of speedo reading.

Increased tyre dia. also affects suspension geometry and increases the pedal effort required for a given brake force. While the difference is small, it's not a great idea to change tyre dia..
 
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